1. Field of the Invention
The invention relates to methods for measuring distribution functions for determining the signal quality in an optical transmission systems. A suitable arrangement makes it possible to measure the statistical properties and main signal parameters. The measurement results can be used for improving the signal quality, for example, for optimizing the dispersion compensation.
Statements concerning the quality of a received binary signal, and consequently also the properties of the transmission system and the transmission link, can be made based on statistical evaluations and measurements of distribution functions. These statements can in turn be used for optimizing the system, for example, for setting an optimum sampling instant, for setting an optimum sampling threshold, or for dispersion compensation.
2. Description of the Related Art
German patent document DE 195 04 896 A1 discloses signal quality monitoring of transparent optical networks in which a random signal sampling is performed. The random amplitude samples obtained in this way are evaluated using a known statistical method. German patent document DE 198 17 078.8 further develops this method by evaluating only the outer edges of a probability density function derived from the measurement results. This allows, for example, the bit error rate to be estimated. However, this method requires very fast sampling and storing capabilities.
U.S. Pat. No. 5,585,954 describes an arrangement for measuring the error rate in the case of different decision-circuit threshold values based on a predetermined pseudo-random sequence, used for determining the transmission characteristic. However, the data transmission has to be interrupted for this purpose. The measured bit error rate is only conditionally suitable for establishing non-linear effects.
In a paper by Hitoshi Takashita and Naoya Henmi Optical Fiber Communication Conference (OFC) 99, San Diego, Calif., FJ 2-1, pp. 149–151, an article titled “A novel data format free bit-by-bit quasi-error monitoring method for optical transport network” describes the use of a receiving circuit with two sampling circuits which have different thresholds. By sampling the binary signal with two different thresholds, a quasi bit error rate is measured and a direct correlation with the bit error rate is established. No further statistical statements are made.